Travelers who passed through three major U.S. airports this month may have been exposed to measles after two cases were confirmed. Measles is a highly contagious virus that, according to the CDC, can infect up to 90% of non-immune people who come in contact with an infected person. The virus is amazingly resilient and can live for up two two hours in airspace after an infected person coughs or sneezes, the CDC says.

The affected airports are in Detroit, Memphis, and Newark, New Jersey. Here’s what you need to know:

Detroit Metro Airport: Passengers who passed through on March 6 are advised to seek treatment if they experience symptoms (more on those below). Per the Detroit Free Press, this includes “Anyone who was in customs or baggage claim in the airport’s north terminal between 2 p.m. and 5 p.m.” More info here.

Newark Liberty Airport: Passengers who passed through terminals B or C on March 12 (between 12:45 p.m. and 9:00 p.m) may have been exposed after a young child with measles traveled there, according to the state’s department of health. Symptoms may appear as late as April 2, the agency says. More info here.

Memphis International Airport: The infected child in Newark departed to Memphis that evening, the agency said. More info here.

Measles Symptoms: Per the NJDOH, “symptoms include rash, high fever, cough, runny nose and red, watery eyes. It can cause serious complications such as pneumonia and encephalitis (swelling of the brain).”

When to worry? Anyone who has not been vaccinated or who has not previously had measles is at risk.